Relays are electrical devices designed to respond to input conditions in a prescribed manner and, after specified conditions are met, to cause contact operation or similar abrupt changes in associated electric control circuits. Input conditions may be electrical, mechanical, thermal, or other quantities or combination of quantities. Electrical inputs include current, voltage, or a combination of current and voltage.
The Institute of Electrical and Electronic Engineers (IEEE) defines a protective relay as a relay whose function is to detect defective lines or apparatus or other electrical system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action. A protective relay operates when an electrical fault--an abnormal, intolerable situation--occurs on electrical transmission or distribution utility lines. A fault is caused by inadvertent, accidental connections between phase wires or from one or more phase wires to ground.
Some natural events that can cause faults include, by way of example, lightning strikes, wind, ice, earthquake, falling trees, or physical contact by animals. Some accidental events that can cause faults include, by way of example, vehicles hitting poles or contacting live equipment, people contacting live equipment, or work crews digging into underground cables. Many faults in an electrical utility system that uses overhead networked lines are one-phase-to-ground faults resulting primarily from lightning-induced transient high voltage and from falling trees and tree limbs.
Faults in an electrical system may provide significant changes in quantities that describe the electrical system. These changes may be used to indicate the presence of the fault and to distinguish between tolerable and intolerable electrical system conditions. Changing quantities include current, voltage or power, power factor or phase angle, power or current direction, impedance, frequency, temperature, physical movements, pressure, and contamination of insulating quantities.
Protective relays are used to sense or determine trouble in an electrical system. Distribution switches or fault protection devices such as circuit breakers and reclosers are used to open and/or isolate problem areas for fault isolation based on the trouble sensed by the protective relay or by their respective controller. Moreover, protective relays may be applied to all parts of an electrical system, including generators, buses, transformers, transmission lines, distribution lines and feeders, motors, capacitor banks, reactors, etc. Typically, protective relays are separate devices that are connected to the electrical system through current and voltage transformers from high system voltages (for example, around several hundred kiloVolts) down to service levels (for example, around several hundred Volts).